The thin film magnetic recording disk in a conventional drive assembly typically consists of a substrate, an underlayer consisting of a thin film of chromium (Cr) or a Cr alloy, a cobalt-based ferromagnetic alloy deposited on the underlayer, and a protective overcoat over the magnetic layer. The word "magnetic" will be used to mean ferromagnetic, antiferromagnetic, ferrimagnetic or any other magnetic material suitable for magnetic recording. A variety of disk substrates such as NiP-coated AlMg, glass, glass ceramic, glassy carbon, etc., have been used. The microstructural parameters of the magnetic layer, i.e., crystallographic preferred orientation (PO), grain size, anisotropy and magnetic exchange decoupling between the grains, play key roles in the recording characteristics of the disk. The Cr underlayer is mainly used to control such microstructural parameters as the PO, the unit cell size and grain size of the cobalt-based magnetic alloy.
One variation of the layer structure described above uses a very thin seed layer on a non-metallic substrate to establish an appropriate nucleation base for the underlayer. Various materials have been used or proposed for seed layers, for example, Al, Cr, Ni.sub.3 P, Ta, C, W, FeAl and NiAl. Laughlin, et al., have described use of an NiAl seed layer followed by a Cr underlayer and a CoCrPt magnetic layer. The NiAl seed layer with the Cr underlayer was said to induce the [1010] texture in the magnetic layer. (See "The Control and Characterization of the Crystallographic Texture of Longitudinal Thin Film Recording Media," IEEE Trans. Magnetic. 32(5) Sept. 1996, p.3632).
The PO of the various materials forming the layers on the disk, as discussed herein, is not necessarily an exclusive orientation which may be found in the material, but is merely the most prominent orientation. When the Cr underlayer is sputter deposited at a sufficiently elevated temperature on a NiP-coated AlMg substrate a [200] PO is usually formed. This PO promotes the epitaxial growth of [1120] PO of the hexagonal close-packed (hcp) cobalt (Co) alloy, and thereby improves the magnetic performance of the disk. The [1120] PO refers to a film of hexagonal structure whose (1120) planes are predominantly parallel to the surface of the film. (Likewise the [1010] PO refers to a film of hexagonal structure whose (1010) planes are predominantly parallel to the surface of the film).
Various designs of thin film disks with laminated magnetic layers are known. The typical laminated magnetic layers are cobalt alloys separated by a thin layer of nonmagnetic material such as Cr or Cr alloy. Although the multiple magnetic layers in laminated disks are all typically composed of the same alloy, different materials can be used.
Since longitudinal recording requires that the C-axis be sufficiently oriented in the plane of the substrate, the range of thin film structures which might otherwise be used is restricted. Some QB alloys have certain advantages as described in U.S. Pat. No. 5,523,173, so it is useful to find ways to overcome the PO problem. The '173 patent describes special sputtering conditions which are useful in depositing QB on an AlMg/NiP substrate to pull the C-axis more strongly into the plane of the substrate. The desired orientation of the QB in the '173 patent is [1120] PO in which the C-axis is sufficiently confined in the plane of the substrate to be suitable for longitudinal recording. One of the conditions in the '173 method is approximately 300 volts of negative bias on the substrate during sputtering of the Cr underlayer.
The topography of the surface on which a thin film is deposited can have a significant effect on the way the film nucleates and grows and also upon its characteristics. So called circumferential texture on magnetic disks has been commonly used to influence the inplane magnetic anisotropy for a wide range of magnetic alloys. For longitudinal recording it is sometimes useful to have a higher coercivity (Hc) in the circumferential direction than in the radial direction. The ratio of the circumferential Hc to the radial Hc is called the orientation ratio (OR). For example, Kneller U.S. Pat. No. 4,287,225 states that he was able to obtain uniaxial magnetic anisotropy (i.e. OR&gt;1) using circumferential texture with an amorphous SmCo alloy. Others have shown similar effects with body-centered cubic (bcc) alloys. (See Arnoldussen, et al., "Obliquely Evaporated Iron-Cobalt and Iron-Cobalt-Chromium Thin Film Recording Media," IEEE Trans. Magnetic., vol. Mag-20, No.5, Sept. 1984, p.821). Current disks typically use hexagonal close packed (hcp) cobalt alloys and most (but not all) circumferentially textured disks have an OR&gt;1.
There are potentially conflicting views on the mechanisms behind the relationship between anisotropy and circumferential texture. Arnoldussen, et al., attributed the anisotropy of their disks to an enhancement of shape anisotropy. Others have claimed to have found that there is a preferential alignment of the C-axes of the hcp crystals parallel to the texture lines, but others have failed to find any such alignment. For example, Miyamoto, et al. in the specification of U.S. Pat. No. 5,352,501, filed in Japan on Dec. 21, 1989, appear to claim to have obtained some degree of preferential C-axis alignment along circumferential texture lines using CoCr(X) alloys where X is Ta, Nb, Pt, Pd, Ni, Zr, W, Mo or Hf. On the other hand, using a CoCrTa/Cr thin film circumferential textured disk Kawamoto and Hikami claim to have determined that magnetostriction was the mechanism underlying the anisotropy and that they saw no preferred crystallographic orientation in the plane of the disk. (A. Kawamoto and F. Hikami, "Magnetic anisotropy of sputtered media induced by textured substrate", J.Appl.Phys. 69(8), Apr. 15, 1991).
One factor contributing to shape anisotropy is the acicularity and orientation of the magnetic grains which in Cr/Co(X) disks are influenced by the underlayer grains. In the prior art elongated grains or particles of magnetic material have been used to control the "easy axis" of magnetization by aligning the long axis of the particles with the desired easy axis. Needle-like magnetic particles have been used to achieve in-plane anisotropy in particulate coated disks for several decades. The needle-like magnetic particles can be mixed with a liquid organic binder and poured onto the substrate in the presence of a magnetic field to orient the particles with the long axis in the plane of the disk and along the circumferential direction. This technique is obviously inapplicable to thin film disks. On thin film disks any acicularity of the grains must be inherent in the growth during the deposition process. Teng and Ballard claimed to have observed acicular magnetic grains on thin film disks (Cr/CoCr and Cr/CoNi) with the longer axis substantially aligned with the circumferential texture grooves and attributed the measured anisotropy to this. (E. Teng and N. Ballard, "Anisotropy Induced Signal Waveform Modulation of DC Magnetron Sputtered Thin Films Disks", IEEE Trans.Mag., vol.MAG-22, No.5, Sept.1986). However, it is not clear from Teng and Ballard's experimental procedure how the elongated grains were created and their result may have been due to hidden or unknown parameters. On typical current thin film disks the grains of the hcp magnetic material grow in an essentially cylindrical shape. It is also possible for the grains to grow in random irregular shapes which have no net acicularity.
The surfaces of the substrates have tended to become smoother as the areal density has increased. In 1984 Arnoldussen, et al. used circumferential texture of approximately 50 nm peak-to-valley. Present disks typically will have a peak-to-valley less than 10 nm. A 10 nm texture appears mirror-like to the untrained eye. Special polishing equipment is necessary to achieve circumferential texture this fine such as is described in Jones, et al., U.S. Pat. No. 5,490,809.
Various oxides and nitrides are suggested as dopants for a thin film magnetic layer in Shimizu, et al., U.S. Pat. No. 5,516,547. Silicon oxide, silicon nitride or boron nitride among others are suggested for alloying with CoPtCr to increase the coercivity and signal to noise ratio.